How to Fix a Mirror SyncVar Hook Not Firing on the Host

Mirror's SyncVar hooks are a client-only delivery mechanism: the authoritative side writes the backing field with no hook call. On a host this surprises people because the host is both server and client.

How to fix it

1. Invoke the hook from the server

After assigning the SyncVar on the server, call the hook yourself: health = newVal; OnHealthChanged(oldVal, newVal);. The hook fires automatically only on remote clients.

2. Share the logic in a helper

Move the visual or gameplay reaction into a separate method that both the server setter and the hook call, so behavior is identical regardless of which side applied the change.

3. Check the hook signature

Mirror requires the new two-parameter signature void Hook(T oldValue, T newValue). An old single-parameter hook silently fails to bind and the compiler warning is easy to miss.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.